S3 (a1): affiliate + partner plumbing

Zero out-of-pocket; everything config-flip-ready but DISABLED/organic.

- BOOK IT deep links: web/src/lib/bookLinks.js + affiliateConfig.js
  (all books enabled:false, Impact/Partnerize param shapes documented,
  empty params skipped). Wired into StatStrip BookItTeaser (real anchor
  now) + scan hand-off links. Every book anchor renders
  rel="sponsored noopener noreferrer" (BOOK_LINK_REL).
- Best-price marker: slateAdapter.detectBestBook (only when >=2 books
  post the SAME line and prices differ — absent beats wrong) + subtle
  signal-green dot in StatStrip. Slate.groupByGame threads the grouped
  per-book rows (books[]) onto PropRowProp instead of discarding them.
- Partner refs: ?ref=CODE -> vyndr_ref cookie (90d, first-touch,
  PartnerRefCapture in layout) -> signup metadata partner_ref ->
  internal GET /api/partners/report/:code (requireInternalAuth; honest
  zeros + note until the TODO migration in docs/PARTNERS.md adds
  user_profiles.partner_ref — NOT run). Stripe promo-code convention:
  partner code == promotion code, verbatim.
- Tests: +41 (2398 -> 2439, 209 suites); bookItTeaser + vyndrCoreScreens
  invariants updated to the new (stronger) rel contract. Web build exit 0.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app.js

@@ -187,6 +187,9 @@ app.use('/api/content', contentRoutes);
 // the public surface; the Next.js admin route proxies through with
 // the key kept server-side.
 app.use('/api/internal', internalRoutes);
+// A1 S3 — partner attribution report. Internal-key gated (router-level
+// requireInternalAuth); no Next proxy on purpose — never browser-facing.
+app.use('/api/partners', require('./routes/partners'));
 
 // Session 10 — Sentry's Express error handler catches uncaught
 // errors from every route mounted above. Must come AFTER routes but